Optical isolators and methods of manufacture

ABSTRACT

Optical isolators and methods of manufacturing optical isolators are disclosed. The optical isolators are manufactured by directly bonding the parts of the isolators without the use of adhesive or mechanical devices to hold the individual parts together.

The present application is a continuation-in-part application of U.S.patent application Ser. No. 10/139,664, filed by Robert Sabia et al., onMay 2, 2002 entitled “OPTICAL ISOLATORS AND METHODS OF MANUFACTURE,” nowU.S. Pat. No. 6,791,748, the disclosure of which is incorporated hereinby reference in its entirety. The present application thus claims thebenefit of U.S. patent application Ser. No. 10/139,664 under 35 U.S.C.§120.

FIELD OF THE INVENTION

This invention relates to optical isolators and methods of manufacturingoptical isolators. More particularly, the invention relates to directbonding of the materials comprising optical isolators and methods ofperforming such direct bonding.

BACKGROUND OF THE INVENTION

Optical isolators are devices used in optical transmission systems toprevent back reflections in optical fibers. Back reflections can occurin fiber systems when light traveling in the system encounters anirregularity such as a change in refractive index between abuttingmaterials or misalignment of fibers in the system. Back reflectionsresult in reduced performance of the system and sometimes can adverselyaffect the transmission source, typically a laser.

Polarization dependent isolators utilize polarizers such as polarizingglass sheets to sandwich the Faraday rotator. In use, an isolator isdisposed between two optical fibers or lenses such that light travelsthrough a first polarizer, then through the Faraday rotator, and thenthrough the second polarizer. In forward or pass mode operation, theincident light emitted from a light source such as a laser passesthrough the first polarizer. The remaining 50% of the light is thenrotated 45° by the Faraday rotator before passing through a secondpolarizer offset from the first polarizer by 45°, preventing loss ofsignal. Polarized light emerges through the second polarizer. In reverseor blocking mode operation, reflected signal transmitted back throughthe isolator is polarized by the second polarizer before being rotated45° by the direction independent Faraday rotator to a polarization mode90° off from the first polarizer. Thus, no signal is transmitted backinto the laser.

Another type of polarizer design utilizes a single polarizer, wherein inthe pass mode, the emitted signal first passes through garnet and thenthrough polarizer. The returned signal is polarized before passingthrough garnet, which rotates the return signal so that it is out ofphase with emitted signal. This scheme promotes minimal laserinterference. This design is less preferred than the isolator designdescribed above which includes two polarizers, however, this singleisolator design is less expensive than the dual polarizer isolatordesign.

Polarization independent isolators are preferred for applications wherethe incident signal is not already polarized. However, the emitted beamis not polarized. Polarization independent isolators include a Faradayrotator sandwiched between two beam splitters, which can be abirefringent material in wedge or plate form, or a prism with a thinfilm coating. In forward operation, incident light emitted from a laseris polarized by a first beam splitter into two distinct polarizationmodes. Each mode passes through a Faraday rotator and an optionalhalf-wave plate, the latter correcting for the 45° rotation imparted bythe Faraday rotator. The modes are then recombined by a second beamsplitter into a non-polarized emission. In reverse or blocking modeoperation, reflected light transmitted back through the isolator isseparated into two distinct polarization modes by the second beamsplitter. When each mode passes through the Faraday rotator and theoptional half-wave plate, the signals are rotated 90° (due to thedirectional dependence of the half wave plate). When both rotated modesare recombined at the first beam splitter, the combined signal istransmitted 90° from the signal feed, thus preventing transmission(i.e., reflection) back into the laser.

Faraday rotators are typically made by surrounding a piece of garnetcrystal with a magnet to apply magnetic field and make the crystaloptically active. This type of garnet is referred to as non-latching.Another type of Faraday rotator utilizes a permanently magnetized orlatching garnet that does not require an external magnet field.

The various component parts of both polarization dependent (Faradayrotator and polarizers) and polarization independent optical isolators(beam splitters and Faraday rotator) are typically held together byeither mechanical assembly or by epoxy or polymeric adhesives. Alimitation of mechanical assembly includes the introduction of opticalsignal loss due to air gaps that tend to exist between the surfaces ofthe adjacent parts, and the need to align and package individualcomponents after dicing to final dimensions. An alternative methodinvolves adhesive bulk assembly or lamination of large sheets ofmaterial followed by dicing, which avoids costs associated withpackaging of individual, pre-diced parts. However, the adhesive assemblyhas the disadvantage of introducing optical loss when the epoxy oradhesive is in the optical path of the isolator. Another disadvantage ofadhesive assembly is that when the isolator assembly encounterstemperature variations, the epoxy or adhesive can fail due to CTEmismatches and/or temperature dependence of the adhesive's refractiveindex, causing delamination of the components. An additionaldisadvantage of adhesive assembly is that the epoxy may be susceptibleto laser damage, causing optical loss, or, in some cases, catastrophicfailure of the device in high power applications.

It would be desirable to provide an inexpensive and reliable method forbonding together the component parts of optical isolators. Furthermore,it would be desirable to achieve bonding of the isolator componentswithout the use of adhesives or epoxy, while maintaining advantages ofbulk assembly prior to dicing to final dimensions.

SUMMARY OF INVENTION

One embodiment of the invention relates to an optical isolator includinga Faraday rotator bonded to at least one beam splitting element orpolarizer, the bond being formed by an adhesive-free and epoxy-freechemical bond or vacuum bond at a temperature below the Curietemperature of the Faraday rotator. In preferred embodiments, thetemperature is below about 200° C. In some embodiments, the bondincludes a covalent bond and/or a hydrogen bond. In certain embodiments,the bond interface includes lithium, and preferably, lithium is includedin either or both of the Faraday rotator, the beam splitter or thepolarizer.

Certain embodiments of the invention relate to a polarization dependentisolator including a pair of polarizers sandwiching and bonded to aFaraday rotator by chemical or vacuum bonding. Other embodiments relateto a polarization independent isolator including a pair of beamsplitting elements sandwiching and bonded to a Faraday rotator. TheFaraday rotator may include either a latching or a non-latching garnet.The polarization independent isolator may further include a half-waveplate disposed between one of the beam splitters and the Faradayrotator, the half-wave plate being bonded to the Faraday rotator and thebeam splitter by an adhesive-free and epoxy-free chemical or vacuumbond. According to certain embodiments, the Faraday rotator and the beamsplitting element or polarizer include bonding surfaces coated with anantireflective coating, which may comprise silica.

Another embodiment of the invention relates to a method of manufacturingan optical isolator including forming an adhesive-free and epoxy-freechemical or vacuum bond between a Faraday rotator and a beam splittingelement or polarizer. In certain embodiments, a Faraday rotator issandwiched between, and bonded to, a pair of polarizers or beamsplitting elements.

Chemical bonds can be formed between a Faraday rotator having a firstbonding surface and a beam splitting element or polarizer having asecond bonding surface by contacting at least one of the bondingsurfaces with a solution to facilitate chemical bonding between thefirst and second bonding surfaces. Acidic solutions or solutions havinga pH greater than 8 can be used. Hydroxide solutions such as ammoniumhydroxide are examples of high pH solutions that may be used inaccordance with this embodiment. Bonding may be facilitated by providingtermination groups on at least one of the bonding surfaces selected fromthe group including —OH, ≡Si—OH, ═Si—(OH)₂, —Si—(OH)₃ and —O—Si—(OH)₃,and combinations thereof. In preferred embodiments, the majority of thetermination groups includes ═Si—(OH)₂, —Si—(OH)₃ and —O—Si—(OH)₃, andcombinations thereof. Bonding may further be facilitated by includinglithium on at least one of the bonding surfaces.

In certain embodiments, the method may include providing anantireflective coating on at least one of the bonding surfaces, and inpreferred embodiments, the coating comprises silica. In otherembodiments, bonding may involve providing adsorbed hydroxyl groups onat least one of the bonding surfaces. Thereafter, the adsorbed hydroxylgroups at the interface between the bonding surfaces can be eliminated,by for example, heating the bonding surfaces to a temperature less thanthe Curie temperature of the Faraday rotator.

The invention provides a simple, low temperature, and reliable bondingmethod that provides bond strength capable of surviving processingconditions, environmental testing and/or a lifetime of service. Bondingcan occur at temperatures lower than the Curie temperature of theFaraday rotator, preferably below about 200° C., and in some cases lowerthan 100° C. Additional advantages of the invention will be set forth inthe following detailed description. It is to be understood that both theforegoing general description and the following detailed description areexemplary and are intended to provide further explanation of theinvention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an exploded view of an optical isolator according to oneembodiment of the invention;

FIG. 1B is an exploded view of an optical isolator according to anotherembodiment of the invention;

FIG. 2 is an exploded perspective view of layers of material for formingoptical isolators according to one embodiment of the invention;

FIG. 3 is a perspective view of the layers of material in FIG. 2 bondedtogether;

FIG. 4 is a perspective view of the layers of material shown in FIG. 3after the layers have been diced into smaller sections.

DETAILED DESCRIPTION

Before describing several exemplary embodiments of the invention, it isto be understood that the invention is not limited to the details ofconstruction or process steps set forth in the following description.The invention is capable of other embodiments and of being practiced orcarried out in various ways.

According to the present invention, optical isolators are provided whichcan be manufactured by using various methods to directly bond opposingsurfaces of the components that comprise both polarization dependent andpolarization independent optical isolators. As used herein, the terms“direct bonding” and “direct bond” means that bonding between twosurfaces is achieved at the atomic or molecular level, no additionalmaterial exists between the bonding surfaces such as adhesives or epoxy,and the surfaces are bonded without the assistance of fusion of thesurfaces by heating. As used herein, the terms “fusion” or “fusionbonding” refers to processes that involve heating the bonding surfacesand/or the material adjacent the bonding surfaces to the softening ordeformation temperature of the articles bonded. The methods of thepresent invention do not involve the use of adhesives, epoxies or fusionbonding to bond the opposing surfaces together. Instead, the presentinvention utilizes methods that involve forming a direct bond betweenthe surfaces without high temperatures that soften the glass material tothe point of deformation or can damage the Faraday rotator material. Thepresent invention provides bonding methods that provide an impermeable,optically clear seal, resulting in essentially zero distortion of thelight passing between the interface of the bonded surfaces. Thesebonding methods include chemical bonding and vacuum bonding. Theformation of a direct bond between the isolator components allows for animpermeable seal that has the same inherent physical properties as thebulk material surfaces being bonded.

Vacuum bonding involves bringing two clean surfaces into contact in ahigh vacuum, thus forming a bond. Provided that the surfaces are flatand clean, a high vacuum removes adsorbed water and hydrocarbons fromthe surface while preventing the adsorption of such species. Surfacescan be processed and cleaned before being placed in the vacuum, orcleaned in the vacuum via ion milling or other plasma techniques.

Within the microelectronics field, vacuum bonding has been developed forsealing of such materials as single crystal silicon, thermal oxide SiO₂grown on Si, and various metals, as described in U.S. Pat. No.6,153,495, the entire content of which is incorporated herein byreference. Coefficient of thermal expansions (CTE) mismatch betweenmaterials is not an issue because the process can be applied at roomtemperature. Because polished wafers are thin and typically non-flat dueto the Twyman effect, special fixturing can be used to apply pressureevenly across the entire wafer surface to generate appropriate contact.

Another type of bonding process that may be utilized according to thepresent invention involves chemical bonding. The formation of a chemicalbond between two glass or metal surfaces allows for an impermeable sealthat has the same inherent physical properties as the bulk materialbeing bonded. In literature, low-temperature bonding technology has beenreported for bonding soda-lime-silicate glass and for crystalline quartz(see, e.g., A. Sayah, D. Solignac, T. Cueni, “Development of novel lowtemperature bonding technologies for microchip chemical analysisapplications,” Sensors and Actuators, 84 (2000) pp. 103-108 and P.Rangsten, O. Vallin, K. Hermansson, Y. Backlund, “Quartz-to-QuartzDirect bonding,” J. Electrochemical Society, V. 146, N. 3, pp.1104-1105, 1999). Both the Sayah and Rangsten references, the entirecontents of which are incorporated herein by reference, disclose usingacid cleaning techniques. Another article, H. Nakanishi, T. Nishimoto,M. Kani, T. Saitoh, R. Nakamura, T. Yoshida, S. Shoji, “ConditionOptimization, Reliability Evaluation of SiO2—SiO2 HF Bonding and ItsApplication for UV Detection Micro Flow Cell,” Sensors and Actuators, V.83, pp. 136-141, 2000, the entire content of which is incorporatedherein by reference, discloses low-temperature bonding of fused SiO₂ byfirst contacting the bonding surfaces with hydrofluoric acid.

According to one embodiment of the invention, functional groups areprovided on opposing surfaces of the isolator components to be bonded.No adhesives, high temperature pre-treatment or caustic hydrofluoricacid treatments are required prior to bonding the opposing surfaces. Inone embodiment of the invention, a surface treatment of a high pH basesolution such as sodium hydroxide, potassium hydroxide or ammoniumhydroxide is utilized to provide functional groups on the bondingsurfaces of the isolator components. In a preferred embodiment, thesurfaces are first cleaned using a detergent followed by rinsing with anacid solution such as a nitric acid solution to remove particulatecontamination and soluble heavy metals respectively.

According to one embodiment of the invention, the surfaces to be bondedare contacted with a high pH solution, rinsed, pressed into contact andgradually heated to the desired temperature, preferably to a temperaturebelow the Curie temperature of the Faraday rotator material, which istypically known from the material specifications. It is preferable touse a “clean” heat source that does not introduce contaminants orbyproducts to interfere with bonding. Such heat sources include, but arenot limited to, induction heating, microwave heating, radio frequency(RF) heating and electric resistance heating. To enhance bonding, it ispreferred that the surfaces are flat; surface conformity can bedetermined by performing a preliminary cleaning and pressing, intocontact, of the dried surfaces, so as to observe the degree ofinterference fringes. Resulting interference fringes can be measuredaccording to techniques known in the art and interpreted to determinematching flatness. Also, an optical flat or interferometer can be usedto evaluate individual surface flatness.

Preferably, the bonding process of the present invention consists ofmachining each isolator surface to be sealed to an appropriate flatness.Particularly preferred flatness levels are less than about 1 micron androughness levels of less than about 2.0 nm RMS. After polishing, eachsurface is preferably cleaned with an appropriate cleaning process suchas first using a detergent, followed by soaking in a low pH acidicsolution, and finally soaking in a high pH basic solution to generate aclean surface. In embodiments in which the isolator surfaces containsilicon, such cleaning and soaking will provide silicic acid-like, forexample, ≡Si—OH, ═Si—(OH)₂, —Si—(OH)₃ and —O—Si—(OH)₃) terminatedsurface groups. Compared with bonding systems that utilize only a low pHtreatment and rely on hydroxyl terminated surface groups, it is believedthe present invention provides more robust bonding betweensilicon-containing articles for several reasons. While not wishing to bebound by theory, it is believed that larger silicic acid-liketermination groups allow bonding (both hydrogen and covalent) to occurbetween surface groups that extend further away from the surface. Largersurface terminated groups such as ═Si—(OH)₂, —Si—(OH)₃, and —O—Si—(OH)₃extend further from the surface than ≡Si—OH, and these larger groups aremore susceptible to steric movement which promotes better bondingbetween surfaces including these larger groups . Additionally, eachsurface can be considerably rougher and still generate bonding due tothe length in which the ═Si—(OH)₂, —Si—(OH)₃, and —O—Si—(OH)₃termination groups extend from the surface.

In a preferred embodiment, the surfaces are assembled without drying. Alow to moderate load (as low as 1 PSI) is then applied as the surfacesare heated to less than 200° C., for example, between 100-200° C. andpreferably below 100° C. so that adsorbed water evaporates and silicicacid-like surface groups condense to form a covalently-bonded interface.

According to certain embodiments of the invention, as noted above, it isdesirable to provide bonding surfaces that are flat. It is preferred tohave surfaces finished to 5 micron flatness or better, and preferably 1micron flatness or better, on the surfaces to be bonded.

Additional information on a preferred embodiment of chemically bondingglass surfaces may be found in copending U.S. patent applicationentitled, “Direct Bonding of Articles Containing Silicon,” commonlyassigned to the assignee of the present patent application and namingRobert Sabia as inventor, the entire contents of which are incorporatedherein by reference. However, the present invention is not limited tothe chemical bonding methods disclosed in the copending patentapplication, and other chemical bonding techniques and vacuum bondingcan be utilized in accordance with the present invention provided theyare compatible with the materials that make up the isolator components.In addition, bonding in certain embodiments of the present invention maybe enhanced by including lithium on at least one of the bondingsurfaces. Additional information on the incorporation of lithium in oron bonding surfaces may be found in copending and commonly assigned U.S.patent application entitled, “Direct Bonding Methods Using Lithium,” andnaming Robert Sabia, Larry Mann and Dennis Smith as inventors.

The various embodiments of the present invention relate to opticalisolators and a method for manufacturing polarization dependent andindependent isolators by sealing surfaces into contact without an airgap or the use of an adhesive. According to one embodiment, polarizationdependent isolators cores (i.e., sandwich-type structures with theFaraday rotator in the middle) are manufactured by sealing a polarizersuch as Polarcor™ glass to a Faraday rotator such as yttrium ion garnet(YIG) or bismuth iron garnet (BIG) to manufacture polarization dependentisolators. In other embodiments, isolator cores are provided thatinclude a Faraday rotator bonded to single a polarizer or beam splittingelement by chemical bonding.

Referring to FIG. 1A, a polarization dependent isolator core 10 isshown, which includes a pair of polarizers 12, 14 sandwiched to aFaraday rotator 16. The polarizers 12, 14 are bonded to the Faradayrotator by vacuum bonding or chemical bonding techniques as describedabove. If non-latching garnet material is used for the Faraday rotator,one way of biasing the material involves inserting the isolator coreinto a magnet 18 as shown in FIG. 1. Other known method of biasing thenon-latching material may be used. If a latching garnet material is usedfor the Faraday rotator 16, magnet 18 is not required to bias theFaraday rotator 16.

In an alternative embodiment shown in FIG. 1B, polarization independentisolator cores 20 can be manufactured by sandwiching a Faraday rotator26 between a pair of beam splitters 22, 24. Typically, the beamsplitters 22, 24 comprise a birefringent material such as single crystalrutile or yttrium vandanate in the form of a plate or wedge, but othertypes of beam splitters may be used in accordance with the invention.Typically, polarization independent isolators also include a half waveplate 25 disposed between Faraday rotator 26 and beam splitter 24. Theadjacent surfaces that make up the isolator core are bonded togetherusing chemical or vacuum bonding techniques as described above. As inthe previously described embodiment, if non-latching materials are usedto form the Faraday rotator 26, the isolator core 20 is inserted in amagnet 28. However, if latching garnet is used to form the Faradayrotator, the magnet can be eliminated.

In an alternative embodiment of the optical isolator not shown), theoptical isolator can include, bonded to the exposed end of thepolarizers or beam splitters, an optical lens. It is contemplated thateither one or both of the polarizers or beam splitters can include abonded lens. As in the previously described embodiment, the adjacentsurfaces of the polarizer or beam splitter and the lens are bondedtogether using chemical or vacuum bonding techniques as described above.The function of the lens bonded to the polarizer or beam splitter is toassist in collimating the signal through the optical isolator and orassist in focusing the signal that is emitted from the optical isolator.Preferably, the type of lens that can be utilized for this applicationincludes, but is not limited to, gradient index (GRIN) lenses.

FIGS. 2-4 show an example of a manufacturing process that can be used toproduce isolators in accordance with various embodiments of the presentinvention. As shown in FIG. 2, sheets of material that are used tomanufacture the individual layers of the optical isolator core areprocessed and polished to an appropriate flatness to enhance chemical orvacuum bonding. The bonding surfaces preferably should have a flatnessof less than 1 micron. As shown in FIG. 2, a sheet of Faraday rotatormaterial sheet 38 is sandwiched between pair of polarizer or beamsplitter material sheets 32, 34. The Faraday rotator sheet 38 includesbonding surfaces 37, 39, that are polished to an appropriate flatness.Beam splitter or polarizer sheet 32 includes bonding surfaces 31, and33. The bonding surfaces 31 and 33 are polished to an appropriateflatness. The bonding surfaces 31, 33, 37, 39 of each of the sheets arethen cleaned and prepared for either vacuum or chemical bonding. Ifchemical bonding is used, a preferred cleaning solution for chemicalbonding the sheets is ammonium hydroxide. The sheets 32, 38, 34 are thenstacked so that bonding surfaces 31 and 37 are in contact and bondingsurfaces 39 and 33 are in contact to form an isolator core sheet 40. Thebonded sheets may be moderately heated to a temperature below the Curietemperature of the Faraday rotator material to enhance the bonding.After the sheets are bonded into a core sheet 40, the sheet may be dicedinto a plurality of isolator cores 50 as shown in FIG. 4, each of cores50 comprised the sheet materials 32, 34, and 38. Thereafter, theisolator cores 50 can be processed according to techniques known in theart.

It will be understood that the sheets of material 32, 38 and 34 may becoated with antireflective coating, in which case, the outer layer ofthe antireflective coating on each sheet will comprise the bondingsurface. Because of refractive index difference between the materialsthat comprise the isolator core materials (e.g., 1.510 for Polarcor™ and2.35 for bismuth iron garnet), an antireflective (AR) coating in mostcases must exist at the interface between the isolator components. Thus,in practice, bonding is actually performed between an AR coated surfaceof one component (either the rotator or beam splitter or rotator orpolarizer), between two AR coated surfaces wherein all adjoiningsurfaces have an AR coating, or between two surfaces where one surfacehas a full (complete) AR coating and the second surface has a SiO₂surface coating that does not by itself act to limit reflectance butrather assists in bonding. These sandwich structures can also bemanufactured to produce multistage isolation by having multiple Faradayrotator layers, with specific application to high powered lasers.

Without intending to limit the invention in any manner, the presentinvention will be more fully described by the following examples.

EXAMPLE 1

This example demonstrates that Polarcor™ polarizing glass sheetsavailable from Corning, Inc. can be bonded together and survive dicingwithout delamination of the sheets. Two Polarcor™ glass sheetspreviously polished to less than 1 micron flatness are coated with ARcoating [SiO₂—ZrO₂—SiO₂] were pretreated with lithium metal by thermalevaporation under vacuum and heat treated at 200° C. for 24 hours. Thesheets were cleaned with a detergent solution (Microclean CA0₅), rinsedin water and soaked in 10 volume % nitric acid solution for 1 hour. Theacid-soaked samples were rinsed in water and then soaked in a 15 volume% ammonium hydroxide solution for 1 hour. The samples were rinsed again,and the bonding surfaces were maintained in a wet condition and bondedunder about 10 pounds per square inch pressure and a bonding temperatureof 75° C. for 24 hours. The bonded sheets were diced into 2 mm strips,and the sheets did not delaminate during the dicing operation.

EXAMPLE 2

This example demonstrates that isolator cores comprising Polarcor™ glasssheets sandwiched together with a Faraday rotator material sheet can bebonded together and diced into isolator cores without delamination. TwoPolarcor™ glass sheets previously polished to less than 1 micronflatness and AR coated were pretreated by coating with lithium metal bythermal evaporation under vacuum and heat treated at 200° C. for 24hours. The lithium coated Polarcor™ sheets along with a flat, AR-coatedbismuth iron garnet (BIG, purchased from MGC) sheet were cleaned with adetergent solution (Microclean CA05), rinsed in water and soaked in 10volume % nitric acid solution for 1 hour. The acid-soaked samples wererinsed in water and then soaked in a 15 volume % ammonium hydroxidesolution for 1 hour. The samples were rinsed again, and the bondingsurfaces were maintained in a wet condition, stacked so that thePolaror™ sheets sandwiched the Faraday rotator sheet, and bonded underabout 10 pounds per square inch pressure and a bonding temperature of115° C. for 24 hours. The bonded sheets were diced into 2 mm×2 mm corestructures of the type shown in FIG. 4, and the sheets did notdelaminate during the dicing operation.

The present invention is directed to isolator structures and sealing orbond processes that result in a chemical bond or vacuum bond without theuse of an adhesive, epoxy or an air gap between applicable surfaces. Theseal or bond can be achieved at temperatures at or below 100° C., thusmaking the process applicable for latched and unlatched garnets. As isknown in the art, heating of garnet materials above their Curietemperatures should be avoided because the properties of the materialare destroyed by heating above the Curie temperature. A seal or bond canbe achieved for a multi-layered core structure where alternating layersof a polarizer and a Faraday rotator are assembled for high power laserapplications (i.e., multistage isolation with 2,3,4, or more Faradayrotators in series with a polarizer for each rotator.

This sealing or bonding process of the present invention can be utilizedfor assembly of glass, crystalline, and anti-reflectance coated surfacesto each other. Isolators that have components (e.g., polarizers,rotators, half wave plates) independently mounted with air gaps requireanti-reflectance (AR) coatings to prevent back reflection at eachsurface due to differences in refractive index between each material andair. Similar differences exist for bonded interfaces due to thedifference in refractive index between the two materials. Thus,interfaces between such materials may require AR coatings. Typical ARcoatings include a base-layer material used primarily for adhesion(example: SiO₂ for Ar coating of glass surfaces), followed by a materialwhich differs significantly in refractive index from the part beingcoated (examples: ZrO₂, Al₂O₃, Nb₂O₃, etc.), and a SiO₂ outer layer. Inpreferred embodiments, an outer layer including silicon such as SiO₂ ispreferred to facilitate bonding. In some embodiments, an anti-reflectivecoating is included on one of the bonding surfaces, and the otherbonding surface, that forms a bond interface, includes asilicon-containing coating such as silica coating to facilitate bonding.Differences in CTE between the coatings and the coated material canresult in a somewhat stressed AR coating. Accordingly, those of skill inthe art will appreciate that coating designs including a number ofalternating layers should account for refractive index differences fortwo materials and differences in CTE.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover modifications and variationsof this invention provided they come within the scope of the appendedclaims and their equivalents.

1. An optical isolator comprising: a Faraday rotator bonded to at leastone beam splitting element or polarizer to provide a bond interface, thebond being formed by an adhesive-free and epoxy-free chemical bond orvacuum bond including lithium at the bond interface, wherein both theFaraday rotator and the at least one beam splitting element or polarizeroperate to provide optical isolating function, and the bonding surfacescontain silicon.
 2. The optical isolator of claim 1, wherein the bondincludes a covalent bond.
 3. The isolator of claim 1, wherein the bondincludes a hydrogen bond.
 4. The isolator of claim 1, wherein the bondinterface includes lithium in the surface of the Faraday rotator or thebeam splitting element or the polarizer.
 5. The isolator of claim 1,wherein the isolator is a polarization dependent isolator including apair of polarizers sandwiched around and bonded to a Faraday rotator. 6.The isolator of claim 5, wherein the Faraday rotator includes a latchinggarnet.
 7. The isolator of claim 5, wherein the Faraday rotator includesa non-latching garnet.
 8. The isolator of claim 5, further including alens disposed on an external surface of either one or both of thepolarizers, the lenses being bonded to the polarizers by anadhesive-free and epoxy-free chemical or vacuum bond.
 9. The isolator ofclaim 1, wherein the isolator is a polarization independent isolatorincluding a pair of beam splitting elements sandwiched around and bondedto a Faraday rotator.
 10. The isolator of claim 9, wherein the Faradayrotator includes a latching garnet.
 11. The isolator of claim 9, whereinthe Faraday rotator includes a non-latching garnet.
 12. The isolator ofclaim 9, further including a half-wave plate disposed between one of thebeam splitters and the Faraday rotator, the half-wave plate being bondedto the Faraday rotator and the beam splitter by an adhesive-free andepoxy-free chemical or vacuum bond.
 13. The isolator of claim 9, furtherincluding a lens disposed on an external surface of either one or bothof the beam splitters, the lenses being bonded to the beam splitters byan adhesive-free and epoxy-free chemical or vacuum bond.
 14. Theisolator of claim 1, wherein at least one of the Faraday rotator and thebeam splitting element or polarizer include bonding surfaces coated withan antireflective coating.
 15. The isolator of claim 14, wherein theanti-reflective coating includes multiple layers and the outer-mostanti-reflective coating layer includes silicon.
 16. The isolator ofclaim 15, wherein one of the bonding surfaces includes ananti-reflective coating and at least one of the boding surfaces includesa silicon-containing layer to assist with bonding of the surfaces.